To make an iPhone more secure, lock it with a passcode. Starting with iOS 9, iPhone supports passcodes with more than four digits. You can also choose passcodes that contain letters. Configure your iPhone so that it locks itself after a short period of inactivity.
Disable access to certain features, including Siri, Wallet and Reply with Message, on an iPhone's lock screen to prevent unauthorized access to information. Turn on the Find My iPhone feature of your iPhone so that you can locate it when it is missing. This feature lets you erase your data remotely. In addition, disable the Control Center on the iPhone's lock screen to prevent someone from putting the phone in Airplane mode, making it untraceable using the Find My iPhone feature.